The invention relates to a method of controlling a laser in a WDM application, in particular, but not exclusively, a DWDM application.
Lasers used as light sources in WDM (Wavelength Division Multiplexing) applications are each designed to operate at a specific wavelength. This wavelength is set by the laser manufacturer and depends on the temperature at which the laser is operating and the current driving the laser. Thus the manufacturer will usually specify an operating temperature and an operating current which will nominally result in operation at the design wavelength of the laser.
In WDM optical transmission systems the wavelength of the source laser needs to be controlled to within tight limits. Any inaccuracy in wavelength setting will result in cross-signal interference and consequent corruption of traffic in adjacent channels, which the user will naturally want to avoid. In DWDM (Dense Wavelength Division Multiplexing) systems, where the channel spacing may be of the order of 100 GHz or less, the wavelength requirements are particularly stringent and are conventionally satisfied by the use of a laser temperature feedback loop which maintains the laser at the manufacturer's stipulated temperature.
A drawback with this conventional control arrangement, however, is that it is impossible to adequately control the wavelength during power-up and power-down situations, where the discontinuities involved give rise to large fluctuations in wavelength value which can affect performance in the manner already described. This is a particular problem during ALS (Automatic Laser Shutdown) operation, when, as part of a safety measure, the laser is disabled when a break in the downstream optical fibre is suspected. During ALS, it is usual to enable the optical output for a short period on a regular basis (e.g. for 2 seconds every 90 seconds, or thereabouts) in order to check whether the fibre break has been repaired. Although temperature is normally held at its stipulated operating value during this process, because the laser is coupled to a large thermal mass (a cooling/heating element), changes in temperature of the lasing element itself which occur during power-up/power-down cannot be quickly compensated for by the temperature control loop (response time is around 10 seconds), and so wavelength drifts significantly from its stipulated value during ALS operation.
The present invention aims to provide a laser control arrangement which mitigates this drawback.